Just wondering, being Dutch as I am, how would I say I'm from holland?
sum a loca plana?
If you want to use that phrase, you'd need to change loca plana to the abl. pl. form -> locis planis.
However, from an internet search, I found some historical maps from the time of the Roman Empire and they indicated that the people of that region were the Frisii. I also found that the area was called Frisia.
So....you could say
Sum Frisius/Frisia. (adjective denoting nationality)
Sum a Frisia.
I hope this helps.